The material obtained from reduced hen egg white lysozyme after complete air oxidation were fractionated by gel filtration to the enzymically active, native species and to the enzymically inactive form which eluted at an elution volume smaller than that of the native species but greater than that expected for a dimeric form of lysozyme. The yield of the inactive form increased up to 100% when the oxidation of reduced lysozyme was accelerated using cupric ion. This inactive form, shown to contain incorrect dusulfide bonds, can be quantitatively renatured by sulfhydryl-disulfide interchange. In the early phase of oxidation of reduced lysozyme three partially oxidized, active species were trapped by alkylation with 14C-iodoacetate. Isolation and characterization of the radioactive tryptic peptides from each of the 3 active forms, permitted the identification of Cys-6 and Cys-127, Cys-76 and Cys-94, and Cys-80 respectivley as the free sulfhydryl groups in the three incompletely oxidized species. Thus, these active species appear to contain three native disulfide bonds and one open disulfide bond. Trapping, by alkylation, the active species containing sulfhydryl groups formed during the denaturation by sulfhydryldisulfide interchange, of the inactive species and examinations of the radioactive tryptic peptides have suggested that the active species containing three native disulfide bonds and one open disulfide bond can be formed in all possible cases.